Gamma-amino butyric acid (GABA) is an important inhibitory neurotransmitter used by many neurons in the retina. The GABAergic circuitry in the retina, however, is poorly understood. This is because the postsynaptic, GABA-ceptive cells have been difficult to identify. Recently, the GABA-A receptor has been purified and antibodies have been raised against it. These antibodies label many amacrine cells, a few bipolar or interplexiform cells, and a group of large, perhaps Y-like, ganglion cells in the squirrel monkey retina. The goal of this proposal is to use the well studied cat retina, a series of double labeling experiments, and electron microscopy to better identify these cells. 1. Identification of the GABA-ceptive amacrine cells: The size, density, and distribution of the receptor-positive amacrine cells will be examined. To better identify these cells, double labeling experiments will be conducted with the antibody against the receptor and antibodies against neurotransmitters (or their synthetic enzymes) known to exist in amacrine cell populations. These include: GABA, glycine, dopamine (tyrosine hydroxylase), and acetylcholine (Choline acetyltransferase). If receptor- positive interplexiform-like cells exist in the cat retina, their size, density and distribution will also be determined. 2. Identification of the GABA-ceptive ganglion cells: The size, distribution, and density of the receptor-positive ganglion cells in the cat's retina will be determined. Retrograde tracers will be injected into the superior colliculus, dorsal lateral geniculate, or medial terminal nucleus to double label the receptor-positive cells and thereby identify their central targets. By determining the number, size, and density of the GABA-ceptive ganglion cells, as well as the destination of their axons, it will be possible to identify these cells in terms of the X,Y, and W cell classification scheme. 3. Synaptic relationships of the GABA-ceptive neurons: Double label experiments with two different fluorophores will be conducted to examine the spatial relationship between the GABAergic telodendria and the receptor bearing processes in the inner plexiform layer. Electron microscopy will be used to study the ultrastructural localization of the receptor and the synaptic contacts may be the receptor-positive cells. Double labeling experiments, in which the receptor-immunoreactivity is labeled with peroxidase and the GABA containing profiles are labeled with colloidal gold, will be used to identify the GABAergic synapses.